1. Field of the Invention
The present invention relates to a system for and a method of controlling drive force distribution for a motor vehicle with drivable front and rear wheels, and more particularly to a system for and a method of controlling the distribution of drive forces from an engine to front and rear wheels according to a continuously variable optimum distribution ratio based on a slip ratio between the wheels and the road surface on which the motor vehicle is running.
2. Description of the Relevant Art
There are known part-time four-wheel drive motor vehicles which are usually propelled in a two-wheel drive mode by driving front or rear wheels only and which can be propelled as desired in a four-wheel drive mode with the front and rear wheels both being postively driven.
Japanese Laid-Open Patent Publication No. 62-7007 (published: Feb. 14, 1987) discloses a drive mode selector device for switching between two-wheel and four-wheel drive modes for a part-time four-wheel drive motor vehicle that is based on and converted from a two-wheel drive motor vehicle known as a front-engine front-wheel drive motor vehicle. The disclosed drive mode selector device controls an electromagnetic clutch to connect the front and rear wheels for switching from the two-wheel drive mode to the four-wheel drive mode when certain conditions such as slippage of the wheels to the road are met. The earlier drive mode selector device however does not determine an optimum distribution ratio continuously variably according to the degree of slippage between the wheels and the road or the condition of the road surface on which the motor vehicle is running, and hence does not distribute the drive forces from the engine to the front and rear wheels according to such a distribution ratio.